The invention relates to a humidity meter or a hygrometer which provides a DC potential proportional to the measured humidity, and in particular, relates to a meter which improves a linear relationship between humidity and an output DC potential.
Conventionally, a humidity meter adapted for use in various kinds of electronic apparatuses such as copying machines and printers is known. The humidity meter uses an impedance variable humidity sensor whose impedance varies with changing humidity. Since the impedance variable humidity sensor has a characteristic that the impedance thereof varies exponentially with respect to the relative humidity, nonlinearity of the humidity sensor must be corrected in order to linearize the humidity-output voltage characteristic when put in actual use. It is common to use a logarithmic amplifier as a linearizing means. More recently known, is an inexpensive linearizing technique using no logarithmic amplifier such as disclosed in U.S. Pat. Nos. 5,065,625 and 5,317,274. In these conventional art devices, the relationship between the base voltage and emitter current of a transistor serving as a voltage control variable impedance element is exponential and that the impedance thereof varies exponentially. That is, nonlinearity of the humidity sensor is corrected by the aforementioned characteristic of the transistor.
However, the aforementioned conventional humidity meters have the following problems.
Since the impedance of the transistor is controlled by the integrated output of an integrating circuit, the base-emitter voltage of the transistor is insufficient when the integrated output, i.e., in the low humidity range. As a result, impedance control of the transistor is not satisfactory, thereby resulting in insufficient linearization of the relative humidity-output voltage characteristic.
In a case where a diode is connected to the emitter of the transistor in series in order to compensate the unsatisfactory nonlinearity of the transistor, a forward voltage of the diode is applied which is unnegligible. Therefore, the insufficient linearization of humidity meters in the range of a small integrated output is unresolved.